The invention relates to a device for controlling the pressure in a hydraulic pressure system, with a hydraulic pump connected to the pressure system on the delivery side and to a hydraulic reservoir on the suction side and with a suction throttle valve controlling the connection between hydraulic reservoir and hydraulic pump.
An arrangement of this generic type is disclosed in German Offenlegungsschrift 3,734,928, in which a suction throttle valve continuously varies the throttle resistance in the suction line, which can also be closed off completely, if appropriate. It is thus possible to control the delivery capacity of the pump, or to cut off the feed of pumping medium while the pump continues to run. A particular advantage of this arrangement is that, with the suction throttle valve closed, the pump works against only a very low resistance, and in all events when a return of the pumping medium from the pressure system is prevented by non-return valves in the delivery line of the pump.
According to German Offenlegungsschrift 3,734,928, an external control of the suction throttle valve is provided. No detailed particulars are given, however. Furthermore, German Offenlegungsschrift 2, 546,600 also shows a pump with suction-flow control. In this device the pressure of the pressure system connected to the pump is used to shift the suction-flow valve between its closed position and its open position. A control-slide arrangement transmits the pressure to a piston connected to the suction-flow valve, in such a way that the valve body of the suction-flow valve is pushed into the closed position. When the piston is relieved of pressure, an opening spring can then push the valve body together with the piston back into the open position again. To allow an especially reliable closing of the suction-flow valve, its valve body is so arranged that, during the closing stroke, it moves in the direction of flow of the suction flow. When the closed position is reached, a vacuum is generated on the outlet side of the suction-flow valve by the continued running of the pump, which loads the valve body of the suction-flow valve in the closed direction.
German Offenlegungsschrift 3,306,025 shows a rotary compressor with a suction throttle valve which is controlled as a function of the pressure o the delivery side of the rotary compressor. The pressure of the pressure system is conveyed via a control valve to a piston which, in addition to a closing spring, can load the valve body of the suction throttle valve in the closed direction counter to the direction of the suction flow. In order to prevent flutter vibrations in the valve body of the suction throttle valve, a two-point control must be guaranteed by the control valve; that is to say, after the piston additionally loading the valve body of the suction throttle valve in the closed direction has been subjected to pneumatic pressure via the control valve to increase the closing force, a relief of pressure should take place only when a certain pressure drop has occurred in the pressure system.
The publication "Grundlagen der Olhydraulik" ["Fundamentals of Oil Hydraulics"] by W. Backe, Institut fur hydraulische und pneumatische Antriebe und Steuerungen der RWTH Aachen, [Institute for Hydraulic and Pneumatic Drives and Controls of the RWTH Aachen], 1986, pages 7-40 and 7-41, shows a hydraulic pressure system with a pressure accumulator and with a reversing valve which is arranged on the delivery side of the pump feeding the pressure system and which, in its one switching state, connects it to the hydraulic reservoir. At the same time, between the pressure system and the reversing valve there is a non-return valve which, in the latter switching state of the reversing valve, prevents a pressure relief of the pressure system via the reversing valve. The reversing valve is controlled as a function of the pressure in the pressure system, a two-point control being guaranteed by separate pilot control valves. Accordingly, at a relatively high pressure in the pressure system, the reversing valve is switched into its circulation position connecting the delivery side of the pump to the reservoir; only after predetermined pressure drop in the pressure system does the reversing valve thereafter switch from the circulation position into the position connecting the pump to the pressure system. A fundamental disadvantage of an arrangement of this type is that, even during circulation, the pump works against a comparatively high resistance and therefore requires a relatively high power. In addition, the pumping medium can be heated substantially during circulation.
The object of the invention is, therefore, to provide a pressure control device which can be made with a low outlay in terms of construction, and which can be integrated with the pump in a simple way.
According to the invention, this object is achieved by providing the suction throttle valve with a slide-like closing member biased in an open position by a spring means A piston is loaded on one side by the pressure in the pressure system counter to the force of the spring means and on the other side by the pressure or vacuum prevailing between the hydraulic pump and the suction throttle valve. The suction throttle valve according to the invention can therefore be constructed in a simple way, similarly to conventional slide valves.
At the same time, an especially good switching behavior is guaranteed; that is, when the pressure in the hydraulic pressure system falls below a lower pressure threshold value the pump introduces pressure medium into this system until an upper pressure threshold value is reached. During the transmission of hydraulic medium into the hydraulic pressure system (with the suction throttle valve open), the vacuum occurring relative to the hydraulic reservoir between the suction throttle valve and the pump has a vanishing value. Accordingly, the instant at which the suction throttle valve closes is determined virtually solely by the pressure in the hydraulic pressure system. The suction throttle valve therefore closes as soon as this pressure, or the resulting force on the closing member in the closing direction, overcomes the force of the spring means acting in the opening direction of the closing member. Because the pump continues to run after closing of the suction throttle valve, a higher vacuum is established between the suction throttle valve and the pump, which acts in the closing direction of the closing member. As a result, the suction throttle valve can open again only when the pressure loading the closing member with a closing effect has fallen in the hydraulic pressure system so far that the force of the spring loading the closing member in the opening direction is sufficient to overcome the sum of the forces which act in the closing direction of the closing member, and which are generated by the pressure in the hydraulic pressure system on the one hand and by the vacuum on the other hand.
Utilizing the vacuum thus provides a reproducible limited hysteresis in the switching behavior of the throttle valve, that is to say a two-point control is guaranteed automatically, without additional control valves.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings